解决方案如下：
1.Close Visual Studio.
2.Run regedit.
3.Hop to HKEY_CURRENT_USER\Software\Trolltech\Versions (如果没有versions，新建一个)
4.Add a new key named "4.7.4" or whatever version you have.
5.Under the new key, create a new string "InstallDir" valued with "c:\Qt\4.7.4":
6.Run Visual Studio
7.Click Qt -> Qt Options.
The new version should be there.